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An Introduction to Cellular Security

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1 An Introduction to Cellular Security
Joshua Franklin

2 Last Changed: License Intro Creative Commons: Attribution, Share-Alike

3 Introduction Cellular networks are a dense subject
This is not a deep dive The standards are large, complicated documents Involves physics, telecommunications, politics, geography, security... We will discuss older cellular networks first and build upon this knowledge The GSM, UMTS, and LTE standards are more or less backwards compatible Major consideration during standards development

4 Who Am I? Joshua Franklin
Intro Joshua Franklin I hold a Masters in Information Security and Assurance from George Mason Graduate work focused on mobile operating systems I work in election and mobile security

5 Learning Objectives Intro Become familiar with the GSM, UMTS, and LTE family of cellular standards Be introduced to spectrum allocation and antennas Learn the security architecture of cellular networks Be introduced to how cellular networks have been hacked in the past We will deeply explore LTE security while only touching on GSM and UMTS. LTE is the new standard moving forward (a.k.a., the new hotness). Previous cellular standards are being phased out.

6 Excluded Topics This class does not cover: Wireless physics
Intro This class does not cover: Wireless physics Ancient wireless networks (AMPS, IMS, smoke signals ) Wired systems (PSTN/POTS/DSL) Standards other GSM, UMTS, and LTE CDMA2000, EV-DO, WiMax In-depth discussion of GPRS, EDGE, and HSPA variants SMS and MMS (text messaging) Mobile operating systems (iOS, Android, Windows Phone) QoS , Mobility management, and VoLTE Internetwork connections Warning: This class is U.S.-centric but the standards are used worldwide. The network operators, frequencies, and implementations vary.

7 Books Wireless Crash Course 3rd edition LTE Security 2nd edition
Intro Wireless Crash Course 3rd edition LTE Security 2nd edition LTE-Advanced for Mobile Broadband - 2nd edition Easy Mode Intermediate God Mode Note: Many papers and presentations were also useful and are cited inline, but check the last slide for a complete listing.

8 Terminology Cellular standards use jargon and abbreviations frequently
Intro Cellular standards use jargon and abbreviations frequently LTE, EPS, BTS, K[ASME] Nested acronyms are common GERAN = GPRS Evolution Radio Access Network LTE is often referred to as Evolved Packet System (EPS) in technical situations Learn to be comfortable with the jargon and acronyms There is an associated glossary and cheatsheet If something needs to be added or modified, please let me know Especially to improve the course and associated documentation

9 Prerequisites Basic understanding of networks and network protocols
Intro Basic understanding of networks and network protocols Familiar with core cryptographic concepts Basic knowledge of information security Basic understanding of physics Have made a phone call There are no labs in this class

10 Agenda Wireless spectrum and cellular bands
Intro Wireless spectrum and cellular bands Important cellular concepts Overview of cellular standards Discussion of the following for GSM, UMTS, and LTE: Network components Security architecture (hardware tokens, authentication, cryptography) Threats to these technologies Notable attacks SIM Hacking Baseband Hacking Femtocells

11 What is LTE LTE is Long Term Evolution
Intro LTE is Long Term Evolution Fourth generation cellular technology standard from the 3rd Generation Partnership Project (3GPP) Deployed worldwide and installations are increasing All implementations must meet baseline requirements Increased Speed Multiple Antennas (i.e., MIMO) IP-based network (All circuits are gone/fried!) New air interface: OFDMA (Orthogonal Frequency-Division Multiple Access) Also includes duplexing, timing, carrier spacing, coding... LTE is always evolving and 3GPP often drops new “releases” This class is modeled around LTE-Advanced, but we won’t dig deep enough to tell

12 Cellular Network Operators
Intro Telecommunications company (telco) Purchases spectrum Builds out network (base stations and backhaul network) Verizon, AT&T, T-Mobile, Sprint Mobile Virtual Network Operator (MVNO) Does not have to purchase spectrum Rents the towers but runs a distinct network Cricket, Ting, MetroPCS, ...

13 Radio Frequency Spectrum
Describes a range of frequencies of electromagnetic waves used for communication and other purposes RF energy is alternating current that, when channeled into an antenna, generates a specific electromagnetic field. This field is can be used for wireless communication Cellular spectrum ranges from 300 MHz to 3 GHz

14 EM Spectrum Thanks to Wikipedia Spectrum
Source: Bidell, p 49 Thanks to Wikipedia

15 Wireless Spectrum From an interactive map available via the FCC
Source and interactive diagram available at From an interactive map available via the FCC

16 Popular Cellular Bands
Spectrum 700 Mhz Band (Blocks A - E) Considered uniquely useful to cellular activities Verizon, US Cellular, AT&T and others own various portions Will be used for 4G Includes reserved spectrum for public safety 850 MHz Great for cellular service Easily bounces off objects 1900 MHz band (PCS) 2100 MHZ (Blocks A - F) Mostly T-Mobile, but includes Cricket and MetroPCS This information changes periodically as spectrum is purchased & released Penetrates walls

17 Chipset Spectrum In the past, phones have typically been tied to a single carrier A phone’s hardware is tied to a carrier based on many things (like the IMEI), but the major ones are the cellular standard and frequencies the carrier uses Phones are manufactured to work on specific radio frequencies Specific chips needed for a given frequency range, thus chipset Nowadays, phones concurrently operate on many frequencies (and therefore networks) Modern multi-band chips allow a single device to operate on multiple frequency ranges

18 Channel Allocation Spectrum Typically there is a downlink channel and an uplink channel These channels needs to be spaced in frequency sufficiently far so that they do not interfere with each other Uplink Downlink

19 Antenna Spectrum There are 2 main types of antennas, each with unique properties Omnidirectional Emits energy in a spherical radius Directional Emits energy in the shape of the antenna and in the direction and angle at which it is pointed

20 Directional Antenna Designed to radiate in a specific direction
Spectrum Designed to radiate in a specific direction The radiation is focused (see below) There are “panel” direction antennas on the front cover of this presentation

21 Omnidirectional Antenna
Spectrum Designed to radiate in across a specific plane The radiation spreads outward from a center point A donut is a reasonable visual

22 Device Antenna Spectrum There are multiple antenna in your mobile device - although some are shared Designed to transmit and receive at various frequencies Cellular (300 MHz - 3 GHz) WiFi (Primarily 2.4 GHz, 5 GHz) [there are other odd frequencies specified] Bluetooth (2400–2480 MHz) NFC (13.56 MHz)

23 Multiple Antennas Spectrum LTE has a feature called Multiple-Input Multiple-Output (MIMO) Multiple antennas are on the mobile device and are used simultaneously to transmit and receive Can significantly increase throughput Multiple types Spatial diversity Spatial multiplexing Further divided: SISO - Single in, single out SIMO - Single in, multiple out MISO - Multiple in, single out MIMO - Multiple in, multiple out

24 Important Concepts

25 Big Picture Concepts Mobile devices (1) connect to a base station (2) which connects to a backhaul network (3), which connects to the internet (4). 1 2 3 4

26 Network Components Concepts The network between mobile devices and base stations is the Radio Access Network (RAN) This name slightly changes with new standards Base stations are permanent cellular sites housing antennas Base stations and the backhaul network are run by telco, but there are interconnections and shared sites AT&T customers need to be able to contact Verizon (vice versa) Base stations often connect to backhaul via wired technologies (i.e., fiber) Base stations often communicate with each other via wireless

27 Mobile Devices Concepts These are the devices with wireless radios that connect to cell towers Radios are inside phones, tablets, laptops, etc. . . LTE uses the term User Equipment (UE), previously ~ Mobile Station (MS) The parts of the UE we are concerned with: The handset, aka the ME (Mobile Equipment) USIM (Universal SIM) Baseband processor

28 Baseband Typically a separate processor on the phone
Concepts Typically a separate processor on the phone From companies like Qualcom, Infineon, etc. Handles all of the telecommunications-related functions Sends, receives, processes signals Base station and backhaul network communication Has direct access to microphone, speakers… Runs a real time operating system (RTOS) Performance matters! OSs include ThreadX, Qualcomm’s AMSS w/ REX kernel, OKL4 Sometimes shares RAM with application processor (baseband as a modem), sometimes each processor has distinct RAM(shared architecture) In a shared configuration the baseband is often the master May be virtualized

29 Planes of Communication
Concepts Many control systems divide communication into two planes - one for processing information from users and another for how to setup/breakdown the channel and other important functions Think of this similar to how FTP uses two ports TCP port 20 - data TCP port 21 - control Control Plane (CP) A private communication channel that is distinct from data the UE operator can influence Used to send control messages to components Mobile users should not be able to influence this in any way User Plane (UP) signaling Voice and data information Cellular networks use this design extensively User Plane Control Plane

30 Packets and Circuits Concepts Pre-LTE, cellular networks used circuit switching technology for voice LTE uses VoLTE which is VoIP over LTE Not implemented currently, calls fall back to previous networks Data traffic is sent over nearly distinct interconnected packet switching networks GSM first used GPRS, then moved to EDGE UMTS used HSPA technologies including HSPA+ Since LTE is completely IP based, it does not use circuits We’re not there yet, but soon.

31 Network Interconnection
Concepts Circuit switched networks need to be able to connect with packet switched networks and other distinct cellular networks The internet is a good example This is a complex process GPRS (General packet radio service) 2.5G packet switched technology EDGE (Enhanced Data Rates for GSM Evolution) 2.75G packet switched technology HSPA (High Speed Packet Access) 3.5/3.75 packet switched data technology There were a few quick iterations on this technology, thus “variants”

32 Attachment, Handoff, & Paging
Concepts The first step in a mobile device connecting to a network is referred to as network attachment Mobile devices request network access to a base station, which passes this request onto the backhaul network Authentication of the mobile device is then performed If a mobile device is moving (such as on a freeway) a call will need to be transferred from one base station to another This is called handoff This is a very common, yet is complex, process Paging is the process of how a backhaul network locates and directs calls a mobile device Base stations provide a list of active devices to the backhaul

33 Connection Management
Concepts EPS Connection Management (ECM) UE related information is released after a certain period of time without use or connection ECM-states ECM-CONNECTED ECM-IDLE TS for more information

34 Subscriber Identity IMSI GSM, UMTS, and LTE all contain a unique ID for a cellular subscriber International Mobile Subscriber Identity (IMSI) 15 digit number stored on the SIM Consists of 3 values: MCC, MNC, and MSIN Possibly a software version (SV) appended (IMSI-SV) Mobile Country Code (MCC) - Identifies the country Mobile Network Code (MNC) - Identifies the network Mobile Subscriber ID number (MSIN) - Identifies a user Temporary identities also exist Temporary Mobile Subscriber Identity (TMSI) Globally Unique UE Identity (GUTI) This information is stored on the SIM/USIM Mobile Subscriber ISDN Number (MSISDN) – The phone number, which is distinct from the MSIN

35 IMSI Example 310150123456789 Mobile Network Code Mobile Country Code
Subscriber ID The MNC may be 2 or 3 digits, depending on region. 3 is common in the USA while 2 is common in Europe. Thanks to Wikipedia for the sample IMSI

36 Terminal Identity IMSI GSM, UMTS, and LTE all contain a unique ID for a terminal ME/UE International Mobile Equipment Identity (IMEI) It is16 digits with the first 14 indicating equipment identity The last 2 indicates software version (SV) Referred to as IMEISV Dial *#06# to display your IMEI Illegal in some countries to change a phone’s IMEI

37 SIM Cards A removable hardware token used for GSM, UMTS, and LTE
Verizon is changing to LTE and is also using the hardware token Over 7 billion SIMs in circulation Houses a processor and runs an OS Java Card runs atop the OS, which is a type of Java Virtual Machine (JVM) for applications Stores cryptographic keys and sometimes SMSs and contacts SIM application toolkit (STK) is used to create mobile applications SIMs are deprecated – the modern term is USIM The USIM runs atop the UICC which is the physical card

38 SIM Card Full-size SIM Micro-SIM Mini-SIM Nano-SIM
From left to right, we are only removing plastic. The integrated circuit remains static. Thanks to Wikipedia

39 Threats to Cellular Networks
Concepts The communication medium is open and accessible by all – Jamming and femtocells SIM Cloning Copying a phone’s unique information to steal another customer’s service Cloning is not as common today Threats to Privacy Cellular networks need, or be able to quickly locate, where the a mobile device is at all times Battery life Pay phones don’t need to be charged once a day Mobile network operators have a large and complex network to defend

40 Jamming Cellular devices send information via radio transmissions
Concepts Cellular devices send information via radio transmissions Interrupting these transmissions is called jamming It is possible to jam a large frequency range, such as all GSM traffic in an area, or only specific frequencies, like those used for control signals 3GPP standards state that jamming attacks are outside of their threat model You can buy jammers online, and depending on the range and power requirements, they can be quite cheap Beware the wrath of the FCC, other three letter agencies, and your local law enforcement

41 Femtocells Concepts Femtocells are small extensions of the cellular network - often for personal or business use Technically the standard refers to them as Home Node B (HeNB) Limited range and relatively affordable They may be provided by telcos if requested and of course you pay for this convenience The purchaser (often the user) does not have full administrative control of the device, similar to set-top boxes The purchaser has complete physical access These devices introduce many new threats Customers retain physical control and can perform offline attacks Attacks on the core network through the femtocell Jamming requires less power because an attacker can be closer Attackers can quickly set one up in new location to attract UEs

42 Cellular Standards

43 3GPP An international standards body
Evolves and/or standardizes GSM, UMTS, LTE among others From their page: The 3rd Generation Partnership Project (3GPP) unites [Six] telecommunications standard development organizations (ARIB, ATIS, CCSA, ETSI, TTA, TTC), known as “Organizational Partners” and provides their members with a stable environment to produce the highly successful Reports and Specifications that define 3GPP technologies We will primarily discuss 3GPP standards Other standards exist from a distinct standards body known as 3GPP2 CMDA2000 and the now deprecated UMB

44 Major Standards Multiple standards bodies involved
Standards grow and evolve from one another GSM CDMA UMTS EV-DO WiMAX LTE

45 Cellular Standards 3GPP Circuit Switched 3GPP Packet Switched 3GPP2
Wimax Forum Generation 2G GSM cdma One GPRS 2.5G 2.75G EDGE 3G CDMA 2000 UMTS CDMA EV-DO 3.5G HSPA/+ 4G LTE UMB WiMAX

46 A Note on 3GPP LTE is a 3GPP specification
Standards LTE is a 3GPP specification Therefore we will be discussing 3GPP specifications in depth We will introduce GSM and associated security issues We will then build on these concepts from GSM to UMTS to LTE Packet switched technologies will be discussed as well 3GPP2 and WiMax Forum standards are not included

47 LTE Security Architecture
Standards The primary 3GPP standard governing LTE is TS Get to know it Other standards exist and are referenced I link to the overarching page for the standard so the most recent version of the standard is attainable Download and extract the zip for the word document

48 GSM

49 GSM Global System for Mobile Communications 2G digital voice
Air interface: TDMA Multiple users on the same channel Operates at various spectrums worldwide There are 4 separate systems: Base station subsystem (BSS) Network subsystem (NSS) Operations and support subsystem (OSS) Mobile station subsystem (MSS) Each subsystem has a distinct purpose

50 GSM Component Description
Mobile station subsystem (MSS) Mobile handset and SIM The base station subsystem BSS consists of a controller and transceiver Base station transceiver (BTS) is the cell tower Base station controller (BSC) controls 1 or more BTSs Housed at the Mobile Telephone Switching Office (MTSO) Network subsystem (NSS): MSC (Mobile Switching Center) and MTSO MTSO-switch connects cell network to PSTN MTSO houses the HLR, which supports the AuC Operations and Support (OSS) Manages the network as a whole

51 GSM Architecture Diagram
BTS Operations and Support Subsystem MS MSC Mobile Station Subsystem HLR/AuC BSC Base Station Subsystem Network Subsystem

52 GSM Security Design GSM Meant to achieve equivalent or greater security than wired systems of that time Security mechanisms should not have a negative impact on the system Primary security mechanisms: Subscriber authentication Privacy achieved via temporary identities Encryption of the Radio Area Network and backhaul ME to BTS and BTS to MMC - using a key known as Kc

53 GSM SIM Tamper resistant hardware token
Stores 128-bit key, called Ki, which is used to derive Kc Ki never leaves the card Also stored in AuC Contains key generation software Subscribers are authenticated to the network by proving knowledge of Ki How? The Authentication and Key Agreement (AKA)

54 GSM AKA AKA is a challenge and response authentication protocol
Authentication is not mutual A devices IMSI is sent to the BTS, which is passed to the HLR/AuC The HLR/Auc sends the Kc, 128-bit random number, and an Expected Response (XRES) to the BTS Kc is a session encryption key The BTS passes the random number to the ME The ME uses the Ki and the random number to arrive at Kc and provides the BTS with an SRES The BTS checks if SRES is equal to XRES If so they subscriber is authenticated The BTS provides the ME with an encrypted Temporary Mobile Subscriber Identity (TMSI) Not always encrypted

55 GSM AKA Ladder Diagram ME (with IMSI) SIM (with Ki) Backhaul
AuC (with IMSI, Ki) BTS IMSI IMSI Random Random, XRES, Kc A3(Random, Ki) = SRES A8(Random, Ki) = Kc SRES SRES = XRES?

56 GSM Cryptographic Algorithms
Families of algorithms: A3, A5, and A8 A3 is used for subscriber authentication to derive XRES A5 is used to encrypt data in motion such as radio encryption ME to BTS A5/0 – no encryption A5/1, A5/2, and A5/3 are 64-bit stream ciphers A5/4 is a 128-bit stream cipher An efficient attack exists against A5/2 and it is deprecated A8 is used to derive the 64-bit symmetric key, Kc The final 10 bits are zeroes The A3 and A5 families are non-standardized They only need to be on devices and equipment owned by the carrier (USIM, BTS, backhaul) MILENAGE is provided if needed Note: GPRS and EDGE use different algorithms

57 MILENAGE Set of five cryptographic one-way functions specified by 3GPP
GSM Set of five cryptographic one-way functions specified by 3GPP Usage is optional as telcos can specify their own Block ciphers with 128-bit key GSM, UMTS, and LTE Used during AKA for key and parameter generation We will explore this further during the LTE segment These are the ‘f boxes’ (f1, f2, f3, f4, f5)[Nyberg04]

58 Threats to GSM Cryptography-based
Short 64-bit keys A5/2 efficient attack A5/1 attack with large amounts of plaintext Implementation flaw exists [Hulton08] Weak cipher renegotiation and null cipher attacks possible SIM cloning Man-in-the-Middle (MitM) attack via rogue base station (femtocell) During AKA, the handset cannot authenticate the network Only radio traffic is encrypted - once information is in the backhaul it is cleartext [Hulton08] IMSI sometimes sent in the clear [Hulton08] Some control signaling may be unprotected

59 Notable Attacks Hulton et al, Blackhat 2008 Paget, Defcon 2010
GSM Hulton et al, Blackhat 2008 Showed how to intercept GSM signals with software defined radio Showed a practical method to crack A5/1 (as did Karsten Nohl) Paget, Defcon 2010 Demonstrated a homegrown GSM BTS Intercepted calls


61 UMTS Universal Mobile Telecommunications System 3G digital voice
Air interface: W-CDMA Operates at various spectrums worldwide

62 UMTS Components UMTS Consists of the core network (CN), Universal Terrestrial Radio Access Network (UTRAN), and UE Runs 2G packet switched and 3G circuit switched components concurrently - it looks confusing at first The UTRAN contains: Node B (think of the phone as Node A) Radio Network Controller (RNC) The CN contains: Serving Mobile Switching Center (GMSC) Gateway Mobile Switching Center (GMSC) Serving GPRS support node (SGSN) Gateway GPRS support node (GGSN) Home Location Register/Authentication Center (HLR/AuC) We are not discussing GPRS-related nodes

63 UMTS Architecture Diagram
BTS BSC SMSC GMSC Packet Switched UE GERAN HLR/AuC Circuit Switched UE Node B RNC SGSN GGSN UTRAN Core Network

64 UMTS & GSM Compatibility
UMTS was designed to work concurrently with GSM 2G SIMs were included Much of the terminology is slightly modified BTS -> Node B

65 UMTS Security Design Iterative enhancement on GSM security
Enhanced AKA New confidentiality and integrity cryptographic algorithms Introduction of Network Domain Security for IP-based protocols (NDS/IP) IPSec

66 UMTS Hardware Token The GSM SIM now labeled the USIM
USIM application runs atop the UICC Contains a new hardware protected 128-bit key: K As in GSM, never moves from UICC and HLR/AuC Keys are derived from K as needed HLR/AuC stores an IMSI and K per subscriber

67 UMTS AKA Similar to GSM - challenge & response protocol
UE proves knowledge of a key UE somewhat authenticates the home network Femtocells can still create a fake connection New algorithms (f1 through f5 and f1* through f5*) AKA algorithms are network specific and don’t need to be standardized UE gains assurance that confidentiality key (CK) and integrity key (IK) were generated by the serving network True serving network authentication is not achieved Man in the middle still possible

68 UMTS Cryptography Completely public algorithms
Increased key-length to 128-bits Yay! Two new families of algorithms UMTS Encryption Algorithm 1 and 2 (UEA1, UEA2) UMTS Integrity Algorithm 1 and 2 (UIA1, UIA2) UEA1 and UIA1 are based on KASUMI Block-cipher related to AES UEA2 and U1A2 are based on SNOW 3G Stream cipher

69 UMTS NDS/IP UMTS Provides protection for control-plane traffic, including authentication and anti-replay Enter NDS/IP Typically does not apply to user plane traffic A security domain under control of mobile network operator Certain connections between components may not be protected due to optional requirements in 3GPP standards Can internetwork with external NDS domains

70 Threats to UMTS Cryptography-based Backward compatibility
There are many attacks against KASUMI [Kühn 2001, Dunkelmann and Keller 2008, Jia et al. 2011, Dunkelmann et al. 2010] Attacks against Snow 3G [Brumley et al. 2010, Debraize and Corbella 2009] Backward compatibility When a GSM SIM is used in UMTS only 64-bit keys are used IMSI catchers during AKA process U. Meyer and S. Wetzel, “A man-in-the-middle attack on UMTS,” in ACM WiSec, 2004, pp. 90–97

71 LTE

72 LTE Long Term Evolution 4G data and voice technology
Also known as the Evolved Packet System (EPS) 4G data and voice technology Air interface: OFDMA 3 main components: Evolved U-TRAN (E-UTRAN) - Radio Network Evolved Packet Core (EPC) - Backhaul IP Multimedia Subsystem (IMS) - Extended backhaul functionality Remember: LTE is a completely packet-switched technology for both data and voice LTE currently falls back to older networks for voice (Circuit-switched fallback) VoLTE (voice over LTE) is in the works I’ll likely need to update this bullet in a year

73 LTE Security Requirements 1
EPS shall provide a high level of security Any security lapse in one access technology must not compromise other accesses EPS should provide protection against threats and attacks Appropriate traffic protection measures should be provided EPS shall ensure that unauthorized users cannot establish communications through the system EPS shall allow a network to hide its internal structure from the terminal Security policies shall be under home operator control Security solutions should not interfere with service delivery or handovers in a way noticeable by end users EPS shall provide support lawful interception

74 LTE Security Requirements 2
Rel-99 (or newer) USIM is required for authentication of the user towards EPS USIN shall not be required for re-authentication in handovers (or other changes) between EPS and other 3GPP systems, unless requested by the operator EPS shall support IMS emergency calls EPS shall provide several appropriate levels of user privacy for communication, location and identity Communication contents, origin and destination shall be protected against disclosure to unauthorized parties EPS shall be able to hide user identities from unauthorized parties EPS shall be able to hide user location from unauthorized parties, including another party with which the user is communicating

75 High-Level Threats to LTE
Tracking identity, privacy or devices Jamming handsets or network equipment or other attacks on availability Physical attacks on base stations or network equipment Manipulating control plane or user plane data Threats related to interaction between base stations, or dropping to older standards or other networks Jamming attacks are not within the threat model of LTE

76 LTE Components User equipment (UE) Evolved Node B (eNodeB)
Mobility Management Entity (MME) Serving Gateway (S-GW) Packet Data Network Gateway (P-GW) Home Subscriber Server (HSS)

77 LTE/EPS Architecture Diagram

78 Component Descriptions
LTE User equipment (UE) – The LTE device Evolved Node B (eNodeB or eNB) – An evolved Node B (BTS) E-UTRAN - The radio network that exists between UEs and eNBs Mobility Management Entity (MME) – Primary signaling node (no user traffic). Large variation in functionality including managing/storing UE contexts, creating temporary IDs, sending pages, controlling authentication functions, and selecting the S-GW and P-GWs Serving Gateway (S-GW)- Carries user plane data, anchors UEs for intra-eNB handoffs, and routes information between the P-GW and the E-UTRAN Packet Data Network Gateway (P-GW) – Allocates IP addresses, routes packets, and interconnects with non 3GPP networks Home Subscriber Server (HSS) - This is the master database with the subscriber data Authentication Center (AuC) - Resides within the HSS, maps an IMSI to K, performs cryptographic calculations during AKA IP Multimedia Subsystem (IMS) – Paging, connections to the PSTN, and

79 E-UTRAN & EPC Protocols
LTE eNB MME Green boxes depict the radio protocol layers. White boxes depict the functional entities of the control plane Intercell RRM NAS Security RB Control Idle State Mgmt EPS Bearer Control RRC PDCP S-GW P-GW RLC IP Allocation Mobility Anchor MAC Packet Filtering PHY E-UTRAN EPC Adapted from 3GPP TS

80 Protocol Discussion LTE There are a number of additional capabilities provided by the enB IP header compression of user data stream Selection of an MME at UE attachment when no routing to an MME can be determined from the information provided by the UE Routing of User Plane data towards Serving Gateway Radio Resource Control (RRC) – Transfers NAS messages, AS information may be included, signaling, and ECM Packet Data Convergence Protocol (PDCP) – header compression, radio encryption Radio Link Control (RLC) – Readies packets to be transferred over the air interface Medium Access Control (MAC) – Multiplexing, QoS

81 CP Protocols UE eNB MME NAS Security NAS Security RRC RRC PDCP PDCP


83 Interfaces LTE Interfaces are the communications paths LTE components use to communicate Each one is provided with its own label There may be unique protocols between various interfaces There are many interfaces - we are discussing a subset X2 - eNB to eNB S1-U - eNB to S-GW S1-MME (sometimes S1-C) - eNB to MME S5/S8 - S-GW to P-GW

84 LTE/EPS Interface Diagram

85 LTE Security Mechanisms
Continue to use the USIM hardware module Subscriber and network authentication via AKA Cryptography Algorithms Key hierarchy Protected Interfaces Protected Planes Independent Domains Access Stratum (AS) Non-access Stratum (NAS)

86 LTE Hardware Token The LTE USIM/UICC is identical to UMTS
Contains a new hardware protected 128-bit key: K As in GSM, never moves from UICC and HLR/AuC Keys are derived from K as needed AuC stores an IMSI and K

87 LTE AKA Very similar to GSM and UMTS AKA 2G SIMs are deprecated
Anchored in hardware token (UICC/USIM) 2G SIMs are deprecated They are unable to authenticate to LTE UEs may drop down to UMTS or GSM We will discuss LTE AKA in detail Overall ladder diagram Generation of AKA security parameters Verification within the USIM

88 LTE AKA Discussion UMTS and LTE AKA are extremely similar
Originally specified in TS So much so, the LTE standard doesn’t even fully describe it (See TS ) Largest update to AKA: network separation Prevents a breach on one telco’s network to spill into another’s Network identity is bound to certain keys AKA directly authenticates network identity New key derivation function specified in LTE

89 LTE AKA Ladder Diagram UE (with IMSI) HSS USIM (with K)
AuC (with IMSI, K) eNodeB MME GUTI/IMSI GUTI/IMSI, SN id Generate Authentication Vectors XRES, AUTN, RAND, K[ASME] RAND, AUTN AUTN Verification SRES Generation SRES SRES = XRES? GUTI = Globally Unique Temporary Identity Adatpted from 3GPP TS

90 AVs Generation LTE The authentication vectors (AVs)are necessary to perform AKA They are requested by the MEE Generated by the HSS/AuC LTE Authentication Vector = (XRES || AUTN || RAND || K[ASME]) AK = Anonymity key AUTN = (SQN xor AK || AMF || MAC) MAC = Message authenticate code in this instance AMF = Authentication Management Field CK = Cipher key IK = Integrity key KDF = Key derivation function MAC = A message authentication function SQN = Sequence Number XRES = Expected response SRES = Signed response

91 AVs Generation Diagram
LTE SQN Generate SQN Generate RAND RAND AMF K f1 f2 f3 f4 f5 MAC XRES CK IK AK SQN xor AK KDF SN id Note: SQN and RAND are generated in a non-standardized manner. K ASME Adatpted from 3GPP TS

92 USIM Verification LTE To verify the AVs in the USM, the authentication process is reversed The same functions f1 through f5 are implemented in the USIM If XMAC != MAC then an authentication failure occurs There is a distinct process for this

93 USIM Verification Diagram
LTE AUTN = (SQN xor AK || AMF || MAC) RAND f5 AK SQN K f1 f2 f3 f4 XMAC RES CK IK XMAC = MAC? Adatpted from 3GPP TS

94 Cryptography in LTE Large change to cryptographic key structure
Introduced a new set of intermediate keys Unique keys for each connection/bearer - large complicated hierarchy Similar to UMTS, we have 2 sets of algorithms for confidentiality and integrity EEA1/EIA1 - based on SNOW 3G EEA2/EIA2 - based on AES (USA) EEA3/EIA3 - based on ZUC (China) CP and UP may use different algorithms

95 Key Hierarchy K Stored in USIM Stored in Auc CK, IK Stored in UE
LTE K Stored in USIM Stored in Auc CK, IK Stored in UE Stored in HSS K Stored in UE Stored in MME ASME Stored in UE Stored in eNB eNB K NASenc K NASint K RRCint K RRCenc K NASint K NASenc K Adatpted from 3GPP TS

96 Key Discussion LTE K – The master key. Permanent pre-shared key stored in hardware. Located on USIM and HSS/AuC CK and IK – Cipher key and Integrity key K[ASME] – Local master. The serving network ID (SNid) is used to derive this key in addition to CK and IK. K[eNB] – Used to derive additional keys used in handoff K[NASent] & K[NASint]- Protection of NAS traffic K[RRCent] & K[RRCint] - Protection of RRC traffic

97 LTE Non-Access Stratum
Security-related signaling between UE and the backhaul Algorithm selection occurs between the UE and the MME MME contains a list of confidentiality and integrity algorithms in a priority order NAS negotiation precedes AKA Negotiation begins when an MME sends an integrity protected Security Mode Command to UE Contains evolved key set identifier (eKSI), list of security capabilities and algorithms, IMSI request, and additional cryptographic information The UE responds with an integrity protected encrypted message called the NAS Security Mode Complete containing its IMEI and a MAC of the message

98 LTE NAS Negotiation UE MME NAS Security Mode Command Encrypted with
UE capabilities, list of algorithms, IMEI request, eKSI, cryptographic information NASenc K Protected with NASint K NAS Security Mode Complete (IMESVI, NAS-MAC) LTE Sec, p 136

99 LTE Access Stratum Signaling between UE and eNB
Algorithm selection occurs between these components eNB contains a list of confidentiality and integrity algorithms in a priority order AS and RRC communication occur on the Packet Data Convergence Protocol (PDCP) AS protection is optional LTE Sec, p 134

100 LTE AS Negotiation UE MME AS Security Mode Command
List of algorithms, AS-MAC Encrypted with UPenc K Protected with UPint K AS Security Mode Complete (AS-MAC)

101 Signaling Protection LTE Network components create protected channels for each component that it is communicating with, for example: UE and eNB communicate with a unique key UE and MME communicate with a unique key eNB and S-GW communicate with a unique key NAS security is always setup if a UE is registered to the network AS security is setup as needed A common claim is that LTE is “fully encrypted” Initial radio access and signaling is not, but no data is being transmitted at that point The standard states that ciphering may be provided to RRC-signaling “RRC signaling confidentiality is an operator option.”

102 Handover Unfortunately, UEs are constantly on the move
This causes the need to be able to switch from eNB to eNB, and possibly from network to network The procedures for this are quite complex as keys and other protected information needs to be transferred or renegotiated Cryptographic keys and encryption/integrity algorithms may need to be changed Refer to our LTE Security book for additional details and the relevant 3GPP standard for additional information

103 Security Contexts LTE Security contexts are a collection of security-related information Algorithms, keys, and other parameters Many contexts are defined: NAS and AS Current and non-current Native and mapped Depending on sensitivity they are stored in the USIM or the RAM of the UE LTE Sec, p 134 Extended key hierarchy allows fast key refreshing for intra-LTE handovers • Security context transferred in handovers with GERAN/UTRAN • After completion of HO, possibility for key renewal • Possibility to refresh keys also during long sessions with no handovers

104 Backwards Compatibility
LTE At times LTE service may be lost and a 2G or 3G system may be available Security Contexts are mapped from one system to another A NAS security context is generated if moving to LTE K[ASME] is used to derive GSM/UMTS security contexts if needed Once mapping has occurred - a new native security context is reestablished as soon as possible AKA can be run again as well

105 Lawful Interception LTE Lawful interception mechanisms are built into 3GPP standards Call/message content and related data provided from certain network elements to the law enforcement side Assumes typically that the content appears in clear in the network element End-to-end encryption is still possible if keys are provided No weak algorithms introduced for LI purposes All 3GPP algorithms are publicly known National variations exist Check TS , , and more additional information

106 Research Considerations

107 SIM Hacking SIMs can be locked using a PIN
PIN is required on phone reboot If PIN is not provided a special code from the telco is required (Personal unlocking key = PUK) Stamped on most SIMs is the ICCID (Integrated Circuit Card Identifier 18 digit unique identifier for the SIM SIMs are updated by over the air (OTA) text messages never displayed to a user Rooting SIM Cards, Blackhat 2013 SIM Forensics Exploring the OS of the SIM, looking for data

108 SIM Hacking Metal Contact

109 SIM Hacking Chip Metal Contact
The UICC contains a CPU, ROM, RAM, EEPROM and I/O, and metal contacts. Plastic Card Body Adopted from Anthony, Sebastian in references

110 Femtocells Often runs a Linux distro
To be used maliciously, root access is required Previous femtocell hacks exploit software vulnerabilities and factory reset procedures Phones automatically attach to the tower with the strongest signal, which is typically the closest tower IMSI-catcher – femtocell that is configured to look like a real base station to steal IMSIs from nearby devices Often used by law enforcement IMSIs are important for device/subscriber tracking and call interception Femtocells: A poisonous needle in the operator’s hay stack, Borgaonkar et al at Blackhat 2011 Traffic Interception & Remote Mobile Phone Cloning with a Compromised CDMA Femtocell, Doug DePerry et al Defcon 21

111 HeNB Home eNode Bs (HeNBs) connect to the backhaul via a distinct Security Gateway (SeGW) Tunnel is protected with IPsec A hardware root of trust is included in modern HeNBs Used to assist in secure boot and integrity check Ensures that upon boot, key firmware and other sensitive security parameters are not modified The status of integrity checks are communicated to the SeGW

112 Baseband Hacking Going through baseband, one can attack the cellular software stack and the mobile operating system (i.e., Android, iOS) Often leads to unlocking Some cellular stacks leverage legacy code Often missing ASLR, NX, heap protection Code not publicly available, reverse engineering of leaked binaries necessary Allows injection of packets via the air interface The IMEISV + IMEI often identifies the baseband software version You may need an external clock to assist with timing, as precision is required Notable work includes R.-P. Weinmann 2010, R.-P. Weignmann 2013 and Guillaume Delugre Femtocells: A poisonous needle in the operator’s hay stack, Borgaonkar et al at Blackhat 2011 Traffic Interception & Remote Mobile Phone Cloning with a Compromised CDMA Femtocell, Doug DePerry et al Defcon 21

113 FIN

114 In Conclusion More detailed security information can be found within:
LTE Security book, 3GPP Standards (TS especially), and Various presentations and whitepapers throughout the web: Security Investigation in 4G LTE Wireless Networks Technical Overview of 3GPP LTE There’s a lot more to cellular security than what is contained within this presentation Study up!

115 Questions || Thoughts? – @thejoshpit
I want this presentation to be accurate Please report errors and omissions (acknowledgement will be provided) Many links went dark while developing this presentation External documents and references (other than videos) are also hosted on my personal domain to ensure they live on All links are properly referenced in the final slide Cellular Security - Part 2 will include a much deeper analysis of LTE networking protocols, crypto, IMS, handover, network interconnection, SIM forensics, and SIM/baseband hacking And other requested topics Joshua Franklin josh dot michael dot franklin at gmail

116 Resources & References
[Hulton08] Hulton, Steve, Intercepting GSM Traffic, Black Hat 2008. Paget, Practical Cellphone Spying, Defcon 2010. Nohl, Karsten, Attacking phone privacy, Blackhat 2010. [Borgaonkar11] Borgaonkar, Nico, Redon, Femtocells: a Poisonous Needle in the Operator's Hay Stack. [Perez11] Perez, Pico, A practical attack against GPRS/EDGE/UMTS/HSPA mobile data communications, Black Hat DC 2011. [Nyberg04] Cryptographic Algorithms for UMTS Dr. Maode Ma, Security Investigation in 4G LTE Wireless Networks Muyung, A Technical Overview of 3GPP LTE Agilent, LTE and the Evolution to 4G Wireless: Bonus Material: Security in the LTE-SAE Network, Agilent Anthony, Sebastian, The humble SIM card has finally been hacked: Billions of phones at risk of data theft, premium rate scams Nohl, Karsten, Rooting SIM Cards, Black Hat 2013 Weinmann, R., The Baseband Apocalypse Weinmann, R., Baseband Exploitation in 2013 Guillaume Delugre, Reverse engineering a Qualcomm baseband TS – LTE Security Architecture TS G security; Security architecture TS – Overall description of E-UTRAN

117 Errors and Omissions Thanks to all that helped make this possible by providing feedback and identifying errors and omissions: Tomasz Miklas ph0n3Ix Skrattar Baddkrash Netsecluke Nechered Chloeeeeeeeee Dr. Kerry McKay BuildTheRobots

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